I've been building an audio amplifier very similar to the Class B amplifier described here. I previously had a problem with my amplifiers clipping before the rail, but I have fixed that by adding a few extra gain stages to take some of the load off each of the individual op-amps. My current schematic is attached as an image here. Overall, the circuit is working well enough but there are two peculiarities that I'd like to get ironed out before I put it together on a perfboard.

1) When I have no input, which is a likely circumstance for my use, I hear a lot of noise coming out of the speaker if I turn the gain up sufficiently loud. How can I do a better job of forcing the output to ground given zero (intended) input? Perhaps DC coupling the input instead of AC coupling the input would help this? I don't have much experience with AC coupling.

2) Similarly, when I have an input and I turn the gain up very loud -- over about 6V peak on the speaker -- my output becomes instantly distorted and my gain drops by, easily, a factor of 100. I have made a few measurements and have noticed that this always seems to correspond to the voltage at the anode of D1 dipping below 0 VDC. Any ideas how I can prevent this? I realize that I can lower the gain of the first gain stage to make the overall gain such that it will not cause the voltage at the anode of D1 to dip below 0 VDC, but I'd like to get as much gain as possible out of this amplifier.

I should say that I understand that this is not a perfect audio amplifier, and I don't intend it to be. This is not being used to amplify MUSIC, but rather to amplify a received, fixed frequency audio signal used to detect ground-loops.

Ahh! I'm sorry. The first stage is drawn incorrectly!! I'll update it ASAP!

There we go. Stage 1 is intended to be an A/C coupled voltage follower.

Okay, I can go ahead and add some caps in parallel with my feedback resistors to band-limit the gain. Yes; Stage 2 & 3 have a very large gain. They are intended to amplify a mV level signal to a loud, audible level. Do you have any suggestions so far as where to place the A/C coupling? Are you thinking between gain stages or perhaps before the output stage?

So far as the last stage goes, I know it looks poor in the schematic attached. My version of SPICE does PNP transistors badly and it has made the drawing messy. It is just a Class B output exactly similar to the output stagethe Class B amplifier described on this website.

The PNP transistor in the output stage is not very well driven with the 15k resistor. It is better to split this resistor and add a "bootstrap" capacitor from the output. The output voltage then supplies voltage to take the lower end of the base resistor below -9V on the negative peaks. So in a way, the transistor provides its own drive current! This is a trick used in lots of small audio amplifiers.

Because most op-amps pull-down better than pull-up (because they have NPN transistors internally) it will be better to invert the whole circuit and have the bootstrap arrangement at the top instead.

Thanks so much for the replies! I'm away from my breadboard and SPICE at the moment. When I get back, I'll try to make my schematic look better by flipping the PNP. I've used SPICE a lot in school, but it was OrCad and now that I'm out on my own, it's hard to pony up for the nice schematic editor

Thanks so much for the ideas, JDT! I'll play with that idea as soon as I get back to my breadboard. The feedback cap looks like a very interesting idea! This looks like it will help with my anomalous gain problem immensely. Any ideas as to how to "quiet" my amplifier when it has no input? I realize that a high-gain amplifier will tend to amplify the hell out of any noise that it picks up, but it seems that you all are much more clever than I; Maybe you have some ideas I would never think of...

The 1 meg on the first input is a noise generator unless the current through it is very tiny, like from a jfet or cmos input op amp. Still, I can't think of any audio range amp that can do a gain of 20,000 and not have background noise.

Like the doctor said, if turning the gain way up when there is no input signal makes noise, don't do that.

I seem to be confused. I just looked up resistor noise, and all the sources say a resistor can make noise without any voltage or current applied to it. Look Ma, a free energy source??? White noise being generated by a resistor that has no current flowing through it???

I should say that I understand that this is not a perfect audio amplifier, and I don't intend it to be. This is not being used to amplify MUSIC, but rather to amplify a received, fixed frequency audio signal used to detect ground-loops.

Yay! I can answer a question... I think. If there is a small amount of current running through the 1M due to, among other things, thermal fluctuations, then the large resistance can create a substantial voltage change. I suppose I should have realized this myself. I may have to look at D/C coupling the input because this amplifier is intended to "seek out" audio signals; ie: It is likely to have its gain turned up high with zero input while the "sniffing antenna" is moved around the lab.

Wow. I just noticed how much this noise is being talked about. We'll see what my two cents do for the discussion. This is something that I'd always heard in school: Thermal noise gets amplified by large resistances. In any case, the amplifier used here is half of a TL082. They are JFET amplifiers with an audio-acceptable slew rate.

EDIT: Tubeguy -- sorry, I missed your post last time. The input comes from a ferrite-core antenna that is being used to inductively pick up noise from ground lines around the lab. The expected pickup level is somewhere from 1 mV to 10 mV.

Yay! I can answer a question... I think. If there is a small amount of current running through the 1M due to, among other things, thermal fluctuations, then the large resistance can create a substantial voltage change.

Click to expand...

No...my hint to #12 was to make him realize that Johnson noise is present on all resistors regardless if there is current flowing through them or not. Johnson noise can be modeled as a perfect resistor in series with a voltage source (the voltage source having a particular RMS depending on R, and a uniform power distribution vs frequency).

That 1M resistor is a noise generator all by itself. You are also correct, though, that the current noise, due to the input bias current of the op-amp, will cause additional RMS noise across the resistor.

This is one reason why it is important to band-limit your design. Noise comes from lots of different sources, and spans frequencies from DC to infinity. Why should we amplify everything when we are only interested in the band covered by our input signal?